1. Field of the Invention
The present invention relates to a gas meter for ultrasound measurements in a breathing apparatus of the type having a gas conduit with an inner wall that defines an inlet at a first end of the gas conduit and outlet in a second end of the gas conduit, and having an ultrasound transducer arrangement that measures the speed of sound in a gas flowing through a part of the gas conduit that forms a measurement chamber.
2. Description of the Prior Art
Ultrasonic gas meters for measuring the flow or the composition of a gas are well known in the art and commonly used in breathing apparatuses, such as ventilators and anaesthesia machines. Such a gas meter typically operates by determining the time of flight for an acoustic (ultrasonic) pulse transmitted through the gas flowing through a gas conduit of the breathing apparatus. The time of flight may then be used to determine the flow of the gas if the gas composition is known, or vice versa.
In order to accurately determine the time of flight of the acoustic pulses it is important to avoid the presence of condensation water or other liquids in the immediate surroundings of the acoustic transducers and in the part of the gas conduit through which the acoustic pulses are transmitted, hereinafter referred to as the measurement chamber.
One problem which may occur if condensation water accumulates in the measurement chamber is that it may reflect the acoustic pulses such that the travel distance and hence the time of flight for the pulses is changed. This problem mainly arises in single-transducer arrangements in which a single ultrasonic transducer mounted on one side of the gas conduit is used both to transmit acoustic pulses through the gas and to receive the pulses after being reflected by the inner wall on the opposite side of the gas conduit, or a reflector element arranged on the opposite side of the gas conduit. In such a single-transducer arrangement, condensation water on the inner wall or reflector element may severely deteriorate the accuracy in the determination of gas flow or composition.
Another problem may occur if condensation water is gathered around the ultrasonic transducer itself. Typically, the ultrasonic transducer is mounted in a transducer housing protruding from the gas conduit. If condensation water penetrates the air gap normally existing between the transducer and the transducer housing, the acoustic pulses may propagate through the material of the transducer housing and further on through the gas conduit, giving rise to various undesired effects. This problem mainly arises in double-transducer arrangements in which two ultrasonic transducers, one acting as a transmitter and the other as a receiver, are arranged on opposite sides of the gas conduit. If condensation water penetrates the air gap in both transducer housings, the acoustic pulses may propagate between the transducers via the transducer housings and the gas conduit instead of through the gas. Such a “short-circuiting” of the ultrasound transducers will, of course, result in erroneous time of flight measurements.
Several solutions have been proposed in order to avoid accumulation of condensation water in ultrasonic gas meters.
European patent application EP 0078381 A2 discloses an ultrasonic air flow transducer arrangement for high humidity environments. The transducers are mounted in transducer housings which are heated by heating means in order to maintain the gas inside the housings at a temperature higher than ambient temperature to prevent moisture from forming therein. The arrangement further comprises water absorbing material to prevent condensation water from entering the transducers and affecting gas measurements.
International patent application WO 96/13701 discloses an ultrasonic gas meter which is designed to prevent liquid accumulation in the transducer housings. This is achieved by draining ducts extending generally horizontally between the transducer cavities defined by the transducer housing and the bore through which the gas flows, or vertically from the cavities to the outside of the transducer housing to permit accumulated liquid to be drained by gravity.
Japanese patent application JP 11051723 A discloses an ultrasonic gas meter in which the longitudinal direction of the gas conduit, or gas transmission passage, is inclined to the horizontal direction to keep the inner wall of the gas conduit clean. In this way, impurities are washed away by condensation water flowing down the inner wall of the gas conduit.